To add an opportunistic encryption mode of access to [IEEE802.11], it
is necessary to perform a Diffie-Hellman key exchange during 802.11
authentication and use the resulting pairwise secret with the 4-way
handshake.

It should say:

To add an opportunistic encryption mode of access to [IEEE802.11], it
is necessary to perform a Diffie-Hellman key exchange during 802.11
association and use the resulting pairwise secret with the 4-way
handshake.

Notes:

As stated in Section 4.4, the Diffie-Hellman key exchange is completed in the 802.11 association step and NOT in the 802.11 authentication step: "Once the client and AP have finished 802.11 association, they then complete the Diffie-Hellman key exchange ...".